1. Field of the Invention
The present invention relates to a powder magnetic core, particularly to a powder magnetic core which has high permeability and can therefore be miniaturized.
2. Prior Art
A powder magnetic core has frequently been used in electric components such as a smoothing choke coil on a direct-current output side of a switching regulator, a reactor of an active filter in an inverter controller, and an operating coil of an injector for use in an internal combustion engine.
Moreover, with miniaturization of the electric component, there has been an increasing demand for miniaturization of the powder magnetic core for use in the electric component. Accordingly, there has been a demand for development of a miniaturized powder magnetic core which has excellent magnetic properties such as high permeability.
The powder magnetic core is generally manufactured as follows.
First, a soft magnetic alloy having a predetermined composition is subjected to a mechanical grinding process or an atomization process and a soft magnetic powder is manufactured. Subsequently, the soft magnetic powder is blended and entirely homogeneously mixed with a predetermined amount of an insulating binder formed, for example, of water glass, and a treatment is performed in order to enhance electric resistivity of the powder magnetic core as a manufacturing object. Subsequently, the mixture is charged into a metal mold, and molded with a predetermined pressure so that a green compact of the powder magnetic core is manufactured. Finally, the green compact is subjected to a heat treatment in order to release molding strain accumulated during the molding, and the powder magnetic core as the object is manufactured.
Moreover, to manufacture the powder magnetic core having the high permeability, it is known to be effective to highly densify the powder magnetic core and to increase the volume percentage of the soft magnetic powder in the powder magnetic core. Therefore, in the above-described manufacturing process, for example, during the molding of the green compact, a high molding pressure is applied to the green compact so that the green compact obtains a high density.
There is a problem that, however, it is difficult to manufacture a sufficiently high-permeability powder magnetic core which has an initial permeability, for example, of 125 or more, simply by raising the molding pressure.
An object of the present invention is to solve the above-described problem and to provide a high-permeability powder magnetic core which has an initial permeability of 125 or more.
According to one aspect of the present invention, there is provided a powder magnetic core mainly comprising a soft magnetic powder which contains: 0.5 to 15% by mass of Si; 10% by mass or less of Al; and the balance of Fe and unavoidable impurities and whose apparent density/true density is in a range of 0.4 to 0.55, wherein a volume percentage of the soft magnetic powder is 80% by volume or more, and an initial permeability at 100 kHz is 125 or more.
Moreover, preferably in the powder magnetic core, the soft magnetic powder is manufactured by an atomization process.
Furthermore, in the powder magnetic core of the present invention, the soft magnetic powder is manufactured by homogeneously mixing at least two types of soft magnetic powders which are different from each other in an average particle diameter.